1. Field of the Invention
The present invention relates to iron-based braze filler metal compositions. More specifically, these filler metals are suitable for the manufacture of several different types of heat exchangers and catalytic converters at a significantly lower cost compared to currently known braze filler metal compositions.
2. Description of Related Art
Nickel-based brazing filler metals with a high chromium content are generally used for their corrosion- resistant and heat-resistant properties. For example, nickel-based fillers may be used in the fabrication and repair of equipment and parts required to operate under high-temperature, corrosive, and/or harsh environment. More particularly, nickel-based or cobalt-based braze filler metals can be used for certain types of heat exchangers, such as exhaust gas regulation (EGR) coolers in automotive applications. Filler metals for these applications must have certain properties to be suitable for use. Such properties include resistance to high temperature oxidation; corrosion resistance; good wettability to the base metals; and not causing embrittlement of base metals during brazing.
Several different grades of nickel-based braze filler metals are defined by the American Welding Society (ANSI/AWS A 5.8) standard. Many of these filler metals are used in the fabrication of heat exchangers. For example BNi-2, a nickel-based brazing filler with a nominal composition of Ni-Bal, Cr-7, B-3, Si-4.5, Fe-3 is a well known filler metal capable of producing braze joints with high strength. A major disadvantage of this filler metal is the degradation of the strength of the base metal due to boron diffusion into the base metal (especially in thin sheet metals as in heat exchangers) and erosion of the base metal. Other boron-containing nickel-based filler metals (such as, for example, BNi-1, BNi-1A, BNi-3, BNi-4 and BNi-9) have similar disadvantages due to the high amounts of boron of nearly 3 percent.
In order to overcome the disadvantages of boron diffusion, other alloys without boron have been considered. These were BNi-6 (Ni-10P), BNi-7 (Ni-14Cr-10P) alloys. These alloys contained approximately 10 percent phosphorus and produced joints without the required strength due to brittle phases in the joint. Another boron-free nickel-based braze alloy is BNi-5 (a Ni-Bal; Cr-19, Si-10). While these alloys were excellent in producing joints without the deleterious effect of boron diffusion into the base metal, there were other disadvantages.
U.S. Pat. Nos. 6,203,754 and 6,696,017 teach nickel-based filler metals of the type Ni—Cr—Si—P compositions that meet several of the requirements for brazing heat exchangers and have excellent corrosion resistance. However, a major disadvantage for these and all the above nickel-based filler metals is the high amount expensive nickel content. These alloys contain a minimum of at least 60 percent nickel. More typically they contain nickel in the range of 70 to 90 percent. The nickel content in these alloys increase the cost of braze filler metals and thus increase the cost of the heat exchangers to unattractive levels. Additionally, the supply of nickel raw material in the world market is volatile, and therefore prices are subject to escalation in an unpredictable manner.
Thus, in light of the factors described above, there remains a need in the art for a brazing filler material that meets the specific requirements for heat exchanger environments while eliminating boron diffusion and achieving lower costs.